In the technical field of electronics, demand for high-density mounting has been increasing. Accordingly, in the technical field using a flexible printed circuit board (hereinafter referred to as “FPC”), physical properties etc. that are suitable for the high-density mounting have been desired.
A process of producing the FPC is broadly divided into (1) a step of laminating a metal onto a base film (hereinafter referred to as “metal lamination step”) and (2) a step of forming wiring with a desired pattern on the metal surface (hereinafter referred to as “wiring formation step”). In particular, in the process of producing the FPC for high-density mounting, a small dimensional change of the base film is desired.
In the metal lamination step and the wiring formation step, the dimensional change of the base film is particularly increased in the following stages. (1) In the metal lamination step, before and after a stage of laminating a metal while heating the base film. (2) In the wiring formation step, before and after etching for patterning the metal. Therefore, when the FPC for high-density mounting is produced, desirably, the dimensional change of the base film is small before and after these stages.
In the production of FPCS, a metal layer is laminated by a roll-to-roll processing of a base film with a large width. Therefore, it is desired that the physical properties of the base film be stable across the entire width (the entire width direction), that is, the rate of dimensional change be stable across the entire width of the base film.
A polyimide film containing a polyimide resin as a main component is suitably used as the base film. In the polyimide film used as the base film, various techniques have been proposed in order to control the rate of dimensional change.
For example, Patent Document 1 discloses a polyimide film produced by appropriately selecting monomer materials in which an average coefficient of linear expansion is about 1 to 25 ppm/° C. in the temperature range of about 50° C. to 300° C. and a coefficient of linear expansion ratio (MD/TD) in the machine direction (MD) and the transverse direction (TD) of the polyimide film is about ⅕ to 4 (see p. 1, claim 1; p. 2, lower left, lines 4 to 14; p. 3, lower right, lines 1 to 10; and the like of the document). In Patent Document 1, the dimensional stability of the film itself during heating is improved.
As a technique for controlling the dimensional changes by stretching a polyimide film in at least one direction, Patent Document 2 proposes a method of swelling a precursor film of polyimide containing a residual solvent with a swelling agent, and then stretching the film in at least a uniaxial direction (see claim 1, paragraphs [0007], [0028], and the like of the document). Patent Document 3 proposes a method of swelling a gel film of a polyimide-amic acid ester copolymer with a solvent, and then stretching the film under heating (see paragraphs [0035] to [0038], and the like of the document). Patent Document 4 proposes a method of biaxially stretching a gel film under a specified degree of swelling, thereby controlling the thermal coefficient of linear expansion in the in-plane direction to be 10 ppm/° C. or less (see claim 3, paragraphs [0015], [0023], [0042], [0045] and the like of the document).
Patent Document 5 proposes a method of stretching a resin film (polyimide film) used as a base film layer of a tape for TAB in a uniaxial direction at a ratio in a predetermined range on the basis of the glass transition temperature, and then annealing the film (see claims 4 and 5, paragraphs [0017], [0041], and the like of the document). Patent Document 6 proposes a method of stretching a polyimide film at a ratio of 1.0 to 1.5 in the MD direction and at a ratio of 0.5 to 0.99 in the TD direction in the production of the polyimide film see paragraphs [0021], [0044], and the like of the document). Patent Document 7 proposes a method of applying a zone stretching (a method of stretching in which molecular chains of a raw film are aligned so as to agglomerate in a reed blind shape) on a polyimide film at a temperature of 250° C. or higher (see p. 1, claim 1; p. 2, upper left, line 15 to upper right, line 6; p. 2, lower left, line 7 to p. 3, upper right line 4; and the like of the document).
Furthermore, as a technique for controlling the dimensional changes by specifying a condition during drying (during imidization) using a tenter furnace (a furnace in which both ends in the width direction of a film are fixed to perform heating) (for convenience, referred to as “tenter process technique”), Patent Document 8 proposes a method of sequentially decreasing the distance between the fixed ends of a film in the first half of a heating furnace, and sequentially increasing the distance in the second half of the heating furnace in producing a polyimide film by a tenter process (see paragraphs [0005], [0032], and the like of the document). Patent Document 9 proposes a method of producing a polyimide film wherein when a self-supporting film is being carried in a tenter furnace while both ends of the film are held, the width of the film between the gripping positions is gradually decreased to 0.95 times that of the gripping part during a step of increasing the temperature to 300° C. at which the shrinkage due to drying is almost completed. Thereby, a polyimide film having a coefficient of linear expansion (TD) of 17 to 24 ppm/° C. in the temperature range from 50° C. to 200° C. and a tensile modulus (TD) of 700 kgf/mm2 or more is produced (see paragraphs [0020], [0021], and the like of the document).
However, none of Patent Documents 1 to 9 describes a film disclosed in the present invention in which the coefficient of linear expansion in a direction of the molecular orientation axis and the coefficient of linear expansion in a direction perpendicular to the molecular orientation axis (for convenience, this direction may be referred to as “a perpendicular direction”) satisfy a particular relationship. For example, in some cases, it is difficult to reduce the rate of dimensional change when a metal is continuously laminated on a film or when wiring is formed by etching the metal layer. Furthermore, the amounts of dimensional change are different between an end of the polyimide film and the central portion thereof. Consequently, it may be difficult to stabilize the rate of dimensional change across the entire width of the film.
On the other hand, Patent Documents 10 to 14 disclose polyimide films in which the composition, the thickness, the tensile modulus, and the tear propagation resistance are specified.
However, none of Patent Documents 10 to 14 describes a film disclosed in the present invention in which the tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis satisfy a particular relationship. In addition, these polyimide films are aimed at improving the handleability during mounting on a substrate or the like and the punchability. Therefore, for example, when a metal is continuously laminated on a film or when wiring is formed by etching the metal layer, the generation of dimensional changes may not be satisfactorily suppressed. Furthermore, the amounts of dimensional change are different between an end portion of the polyimide film and the central portion thereof. Consequently, it may be difficult to stabilize the rate of dimensional change across the entire width of the film.
Polyimide films are generally produced by a tenter furnace process in which ends of the film are gripped with clips or pin seats, and the film is transferred through a high-temperature furnace to bake the film. However, when a polyimide film is produced by the tenter furnace process, the same phenomenon as that caused by anisotropy of molecular orientation (generally referred to as “bowing phenomenon”) described in, for example, Non-Patent Documents 1 and 2 occurs in the production process of the polyimide film. Consequently, the anisotropy of molecular orientation is generated at the ends of the film (in particular, a part located within about 50 cm from a film-gripping device). When such anisotropy is exhibited, for example, a difference in coefficient of linear expansion and a difference in dimensional change are generated in the width direction of the film.
The present inventors have found that when the ratio between the coefficient of linear expansion in a direction of the molecular orientation axis of a polyimide film and the coefficient of linear expansion in a direction perpendicular to the molecular orientation axis thereof satisfies a particular relationship, the rate of dimensional change in the case where a metal is continuously laminated on the film or wiring is formed by etching the metal layer is small, or the rate of dimensional change can be stabilized across the entire width of the film.
Furthermore, the present inventors have found that, in a polyimide film in which the ratio between the tear propagation resistance c in a direction of the molecular orientation axis of the polyimide film and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis thereof (for convenience, this direction may be referred to as “a perpendicular direction”) is within a specific range, the rate of dimensional change in the case where a metal is continuously laminated on the film or wiring is formed by etching the metal layer is small, and the rate of dimensional change can be stabilized across the entire width of the film.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 61-264028 (Publication Date: Nov. 21, 1986)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-1804 (Publication Date: Jan. 8, 2002)    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-128811 (Publication Date: May 8, 2003)    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-145561 (Publication Date: May 20, 2003)    Patent Document 5: Japanese Unexamined Patent Application Publication No. 8-174695 (Publication Date: Jul. 9, 1996)    Patent Document 6: Japanese Unexamined Patent Application Publication No. 11-156936 (Publication Date: Jun. 15, 1999)    Patent Document 7: Japanese Unexamined Patent Application Publication No. 63-197628 (Publication Date: Aug. 16, 1998)    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2000-290401 (Publication Date: Oct. 17, 2000)    Patent Document 9: Japanese Unexamined Patent Application Publication No. 2002-179821 (Publication Date: Jun. 26, 2002)    Patent Document 10: Japanese Unexamined Patent Application Publication No. 11-246685 0009    Patent Document 11: Japanese Unexamined Patent Application Publication No. 2000-244083 0010 and 0011    Patent Document 12: Japanese Unexamined Patent Application Publication No. 2000-198969 0009 and 0010    Patent Document 13: Japanese Unexamined Patent Application Publication No. 2000-208563 0008 and 0009    Patent Document 14: Japanese Unexamined Patent Application Publication No. 2000-208564 0008 and 0009    Non-Patent Document 1: Kunisuke Sakamoto, Kobunshi Ronbunshu (Japanese Journal of Polymer Science and Technology) Vol. 48, No. 11, pp. 671-678 (1991)    Non-Patent Document 2: Chisato Nonomura et al., Journal Seikei-Kakou, Vol. 4, No. 5, pp. 312 to 317 (1992)